1. Field of the Invention
The present invention generally relates to atomic magnetometers and, more particularly, atomic magnetometers having negative feedback.
2. Description of the Related Art
In the ubiquitous health (U-Health) age pursuing safety life and healthy life that are the trends of 21 century, development and measurement/analysis techniques of a biomagnetic measurement apparatus, such as magnetocardiography (hereinafter referred to as “MCG”) and magnetoencephalography (hereinafter referred to as “MEG”), based on a technique of an optical-pumping atomic magnetometer that is an ultra-sensitive magnetic sensor (magnetometer) are core techniques for the next-generation ultra-sensitive instant diagnosis system.
In particular, ultra-sensitive magnetic measurement techniques have been widely researched in various fields ranging from pure learning to industry. In a magnetic measurement technique, the direction of application varies depending on a measuring range. Recently, a microfabricated atomic magnetometer was developed by P. Schwindt of NIST. It is expected that researches will be made on the microfabricated atomic magnetometer grafted onto the ubiquitous technology to be applied to portable magnetic resonance imaging, a buried explosive detector, a remote mineral meter, and the like.
A spin-exchange-relaxation-free (SERF) atomic magnetometer has been researched in recent years. The SERF atomic magnetometer may be used at room temperature without a coolant while exhibiting sensitivity equal to that of a SQUID sensor. Accordingly, the SERF atomic magnetometer may be used in all SQUID precise measurement fields that have been searched for a long time. As a result, the development of an atomic magnetometer technique will further improve and advance excellent magnetic measurement application techniques.
In particular, the SERF atomic magnetometer will be widely used in technical development of noncryogenic MCG/MEG measurement. It is expected that world-leading research and development will be made in the atomic magnetometer field by combining the SERF atomic magnetometer technique with a biomagnetic measuring technique, an atomic clock technique, and a spectroscopic technique. Subsequent researches will be widely used in progress of a precise absolute magnetic measurement technique and development of various magnetic-based nondestructive inspection techniques.
An existing technique used in precise magnetic measurement has been searched through various methods such as magnetic resonance (e.g., nuclear magnetic resonance, optical pumping, etc.) and magnetometer using the Hall effects or fluxgate principle. These methods have advantages and disadvantages depending on magnetic measurement methods. A fluxgate magnetometer can be easily manufactured because its measurement method is simple. However, the fluxgate magnetometer is limited in measuring a low magnetic field.
A magnetic field is one of the most basic physical quantities that can be observed everywhere. The magnetic field transfers information on all electromagnetic phenomena. A high-sensitive magnetic measurement technique has been researched with wide attention from pure learning to industry. Currently, a SQUID-based sensor has been used as a high-sensitive magnetic measurement apparatus that is most sensitive to a magnetic field. However, the SQUID-based sensor does not come into wide use due to theoretical measurement limitation and costs required for ultra-low temperature cooling for a superconducting phenomenon and required for maintaining the same.
In order to overcome the above disadvantage, vigorous searches have been made on magnetometers using interaction between light and resonating atoms. Sensitivity of an atomic magnetometer may be equal to or greater than that of a SQUID-based magnetometer. Thus, a biomagnetic filed that can be measured only by a SQUID sensor may be measured by an atomic magnetometer that need not be cooled and maintained. For this reason, a biomagnetic diagnosis technique useful in medical diagnosis such as epilepsy, brain function mapping, myocardial infarction, arrhythmia, and fetal function may come into wide use.
In recent years, various researches have been made theoretically and experimentally on small magnetic field measurement using interaction between atoms and laser.
The Scully Group theoretically calculated a small magnetic field measurement limitation arising from nonlinear magneto-optic effect at a coherent atomic medium. According to the research, it was reported that a limitation of magnetic field measurement sensitivity was 0.6 fT/Hz1/2 when Rb atoms were used.
In 2003, the Romalis Group showed that a magnetic field can be measured with sensitivity of 0.54 fT/Hz1/2 by using a method of detecting Larmor spin precession based on optical pumping.
In 2004, the Hollberg Group of NIST developed a magnetic field measurement sensor having height of 3.9 mm and volume of 12 mm3 using coherent population trapping (CPT). Measured sensitivity of the sensor was about 50 pT/Hz1/2.
In 2006, the Budker Group developed a magnetic field measurement sensor by forming an anti-relaxation coated spherical cell having a diameter of 3 mm. It was reported that measured sensitivity of the sensor was 4 pT/Hz1/2.
Recently, a microfabricated atomic magnetometer was developed by P. Schwindt of NIST. It is expected that researches will be made on the microfabricated atomic magnetometer grafted onto the ubiquitous technology to be applied to portable magnetic resonance imaging, a buried explosive detector, a remote mineral meter, and the like.
Recently, the Romalis Group developed an atomic magnetometer having magnetic field measurement sensitivity of 160 aT/Hz1/2 in a spin exchange relation free (SERF) regime.